1. Technical Field
The present invention relates to an apparatus for inflating an air bag to restrain movement of a vehicle occupant during a collision.
2. Description of the Prior Art
U.S. Pat. No. 4,846,368 discloses an inflator for inflating an air bag to restrain movement of a vehicle occupant during a collision. The inflator includes a quantity of combustible gas generating material contained in a perforated tube. A cylindrical filter is wrapped around the perforated tube. A cylindrical housing extends around and encloses the gas generating material, the perforated tube, and the filter. The housing has openings in its cylindrical wall which direct gas generated by the inflator into the air bag.
In the inflator of U.S. Pat. No. 4,846,368, the pressure of the gas generated in the inflator builds up to a predetermined pressure before the gas is directed into the air bag. This is achieved by a pressure controlling layer of foil disposed on the inner surface of the cylindrical housing. The layer of foil encircles the filter and the gas generating material. Portions of the layer of foil initially block the flow of gas outwardly through the openings in the cylindrical housing. When the pressure of the gas inside the housing builds up to a predetermined pressure, the portions of the foil layer burst to release the gas. A plenum screen is wrapped around the cylindrical outer surface of the filter, radially inward of the layer of foil. The plenum screen creates an annular plenum in which gas generated by burning of the gas generating material can accumulate until the pressure builds up enough to burst the layer of foil.
The plenum screen in the inflator of U.S. Pat. No. 4,846,368 is a layer of mesh screen. The screen has a plurality of wires extending in a first direction interwoven with a plurality of wires extending in a second direction transverse to the first direction. Because the wires are interwoven, the plenum screen does not present a smooth outer surface to the layer of foil. Rather, at the locations where the wires overlap and engage the layer of foil, stress concentrations can occur.